The device works at cryogenic temperatures, which is so cold that all the gasses would freeze on the experiment device. Moreover, before they do so, they would conduct heat from the walls of the chamber to the experiment device and make it hard to cool the device down.
Thus, you need vacuum for being able to cool things down to a very low temperature, and once the device is cold, the presence of the cold elements even improves the vacuum, as the little rest gas you were unable to pump out before will freeze - effect known as cryopumping.
See also
Why must quantum computers be kept near absolute zero?